ring: fix return of new port id on creation

ring: fix return of new port id on creation

The rte_eth_from_rings API allowed the creation of an ethdev port at
runtime using rte_rings as the underlying storage. However, the return
value from this function was either 0 or -1, and these values were never
actually documented in the API documentation. Unfortunately, the programmers
guide doc examples for this API implied that the return value from this
function was the port id of the newly created ethdev.

Since this latter behaviour is more useful - and already implied by the
documentation, this patch changes the return 0 to "return
data->port_id". It also adds in doxygen comments for the function so it
can be correctly documented in the API reference.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>